The present invention pertains to a circuit arrangement for receiving light signals, which are emitted over an open optical path, i.e., in ambient light. The circuit is used to receive light signals which are sent by evaluating the intensity for recognizing changes occurring over a limited time in the structure of a medium which is located in the transmission path. Changes of this kind may be caused by, for example, water drops located in the air of the optical transmission path or on a glass pane which is arranged in the transmission path.
An arrangement for measuring or recognizing the wetting of a wall or plate transparent to a certain radiation was proposed in German Utility Model No. G 93 09 837.5. This arrangement can be used, for example, to detect water drops located as precipitate on the windshield of a motor vehicle.
This device uses an optical arrangement with two light-emitting diodes, between which a light sensor is located, and which are arranged behind the windshield of a motor vehicle. The light-emitting diodes emit push-pull-pulsed light. The light sensor receives part of the light of the light-emitting diodes reflected from the windshield. The windshield reflects the rays emitted in the push-pull mode approximately equally in the dry state, so that the received light from the two light-emitting diodes acts like constant light in the circuit arrangement. The reflection condition, and consequently the signal ratio, change irregularly at short time intervals upon the impact of rain drops. The received light component of one light-emitting diode predominates, and an alternating current is generated in the light sensor in the cycle of the pulse frequency of the light that is emitted by the light-emitting diodes. These changes are evaluated.
Such a device, which is operated in an optically unshielded space, has the problem that intensely varying, undefinable ambient light is present, and this light continuously reaches the light sensor as an interfering light. This light is usually constant light, for example, the light of the sun or of incandescent lamps. However, it may additionally also be received as chopped light, such as the light of gas discharge lamps or a constant light influenced by the travel motion of the motor vehicle. The intensity of the ambient light may change in rapid succession in an undefinable manner during the travel of the motor vehicle, so that no reliable information can be obtained from this light on the nature of the transmission path, for example, the presence of rain drops. If the light sensor is equipped with optical filtering means, this parasitic light is then received at varying intensity and is converted into a corresponding electrical signal component. The converted electrical signal, therefore, always consists of an equisignal component, whose value may vary within a broad range, and a superimposed alternating signal component, which represents the evaluable wanted signal. The level of the equisignal component may be a multiple of the amplitude of the wanted signal. At high intensity, parasitic light may overmodulate the light sensor and/or the receiving amplifier. In this case, the wanted signal cannot be sufficiently separated from the equisignal component and evaluated. Measures are therefore necessary to eliminate the parasitic light.
To support these measures, the light is scanned at a frequency of about 40 kHz. Only insignificant parasitic light components are present in this frequency range.
A light receiver comprises, in general, a light sensor, an amplifier, means for suppressing the effects of parasitic light, and a demodulator for recovering the signal from the converter photocurrent. This signal is evaluated by a synchronous demodulator, and it generates a signal which corresponds, for example, to the frequency of defects in the optical transmission path. Photodiodes are usually used as light sensors.
Such a circuit arrangement has been known from the book "Linear Circuits Data Book 1992," Volume 1, Operational Amplifiers of the Firm of Texas Instruments Incorporated, U.S.A., on page 2-722, FIG. 47. This circuit arrangement is shown in FIG. 1 of the present application for better understanding, and it contains two linear operational amplifiers (precision double operational amplifiers in the CMOS technique of the type TLC 27 M 2 may be used).
The cathode of a photodiode is connected to the inverting input of a first operational amplifier, and the anode is connected to the ground of the circuit. This first operational amplifier is negative feedback-connected only weakly via a first loop, which contains a high-ohm resistor (10 Mohm for example) and has a high current-voltage factor during the transmission of the photocurrent in order to generate a sufficient amplitude of the wanted electric signal even at a low brightness amplitude, which, compared with the parasitic light, the modulated signal light has.
The noninverting input of the first operational amplifier is connected to a reference voltage, whose value may be between zero and the operating voltage of the circuit minus 2 V. Regardless of the intensity of the received light, the operating point of the photodiode is placed with this reference voltage into the stop band. This band guarantees a sufficient overmodulation resistance for the amplitude of the modulated signal light.
Via a second loop, which contains a second operational amplifier connected to a low-pass filter as a noninverting d.c. voltage amplifier, the output signal of the first operational amplifier, which must be fed to a corresponding circuit for evaluation, is additionally negative feedback-connected, by d.c. voltage, substantially more strongly than via the first loop. This negative feedback brings about a compensation of the photocurrent that is generated by the parasitic light in the photodiode.
When light arrives, the blocking resistance of the photodiode decreases in a manner corresponding to the brightness. The change in the photocurrent flowing over the negative feedback into the photodiode brings about an increase in the output voltage in the first operational amplifier. Due to its integrator function, the low-pass filter ensures that only the d.c. voltage component of the output voltage generated by the first operational amplifier is fed back negatively to the cathode of the photodiode via the second loop.
Thus, only the part of the signal current that is generated by the photodiode and whose frequency is above the limit frequency of the low-pass filter is transmitted to the output of the circuit with a high current-voltage factor. The effect of the parasitic light is very extensively suppressed.
However, one drawback of this arrangement is the high ohmic, negative feedback of the first operational amplifier, which is necessary for obtaining a high current-voltage factor in order to convert the small percentage of reflected light of the wanted signal into a sufficiently high signal voltage. At an unfavorable ratio of parasitic light to signal light, which may occur in this case, a high operating voltage is necessary for a sufficient equisignal suppression and overmodulation resistance of the circuit, and such a high operating voltage may be above the voltage available in a motor vehicle. Due to these requirements, this prior-art circuit is not ideally suited for its intended purpose because of the excessively high operating voltage needed, an insufficient transmission frequency, and a high interference susceptibility, which are the consequences of the high ohmic circuitry.
At an unfavorable ratio of signal light to parasitic light, the photodiode of the prior art device must be operated with a bias voltage, at which the photodiode operates in the blocking state. Only a very low signal alternating current is generated in this operating range. The downstream amplifier, therefore, must have a high signal sensitivity in order to convert the very weak signal current into an evaluable signal voltage.
Another problem with the prior art device is the geometric arrangement of the photodiode, the signal line, and the receiving amplifier. Since the photodiode has a high internal resistance in the blocking state, which may reach several hundred kOhm, many parts of the circuit are especially sensitive to the coupling of interference signals, for example, interferences generated by the ignition system of motor vehicles, power line hum, switching impulses, and, in a special case, to the coupled signal from the light-emitting diodes and other interferences which may reach the circuit inductively and/or capacitively due to leakage fields. Electromagnetic shielding and compact design of the photodiode and of the amplifier are therefore necessary, and this may even make it necessary to arrange the light inlet opening at the photodiode behind a shield made of a fine wire-mesh screen. Such a compact and shielded design is expensive and is highly disadvantageous from the viewpoint of design and layout (at the windshield of a motor vehicle).